Advances in semiconductor manufacturing technologies have resulted in dramatically increased circuit packing densities and higher speeds of operation. In order to achieve such increased densities a wide variety of evolutionary changes have taken place with respect to semiconductor processing techniques and semiconductor device structures over the years.
Some of the more recent changes in semiconductor processing and device structures include gate replacement, where polysilicon gate electrode structures are removed after source/drain formation, and a high-k gate dielectric layer with a metal gate electrode are provided in their place; and source/drain terminal formation by epitaxial growth of p+ and n+ layers. In connection with source/drain terminal formation by epitaxial growth, some modern processes provide recesses adjacent a gate electrode structure, prior to gate replacement, in which a first conductivity type epitaxial layer is formed. Similarly, such processes may provide raised source/drain terminals adjacent a different gate electrode structure prior to gate replacement. These raised source/drain terminals may be formed by epitaxial growth without first forming a recess.
In addition to the formation of transistors, semiconductor processes are also used to create other active and passive electrical components. An example of another active electrical component is the diode. Diode structures result when semiconductor material of a first conductivity type and a second conductivity type are formed adjacent to each other. Given the appropriate relative doping profiles, such diodes may have the electrical performance characteristics that put them in a class of diodes referred to as Zener diodes.
A Zener diode is a special kind of diode that allows current to flow in the forward direction just as an ideal diode does, and also allows current to flow in the reverse direction when the voltage is above the breakdown voltage without suffering the thermal damage that can be experienced by non-Zener diodes operating in breakdown mode. Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits. The operation and electrical characteristics of a Zener diode depends on the heavy doping of its p-n junction to allow electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. Some conventionally formed Zener diodes, made in processes that integrate the formation of field effect transistors and Zener diodes, require the use the interface of a lateral n+ and p+ structure to form the Zener diode junction. Such conventional a Zener diode structure requires a large amount of die area and tends to exhibit undesirably high leakage current.
What is needed are diode structures that are compatible with the formation of epitaxially formed, raised source/drain terminals in gate-last, high-k MOSFET integrated circuits.